An exhaust gas sensor (e.g., exhaust oxygen sensor) may be positioned in an exhaust system of a vehicle and operated to provide indications of various exhaust gas constituents. In one example, the exhaust gas sensor may be used to detect an air-fuel ratio of exhaust gas exhausted from an internal combustion engine of the vehicle. The exhaust gas sensor readings may then be used to control operation of the internal combustion engine to propel the vehicle. In another example, outputs of the exhaust gas sensor may be used to estimate a water content in the exhaust gas. Water content estimated using the exhaust gas oxygen sensor may be used to infer an ambient humidity during engine operation. Further still, the water content may be used to infer an alcohol content of a fuel burned in the engine. Under select conditions, the exhaust gas sensor may be operated as a variable voltage (VVs) oxygen sensor in order to more accurately determine exhaust water content. When operating in the VVs mode, a reference voltage of the exhaust gas sensor is increased from a lower, base voltage (e.g., approximately 450 my) to a higher, target voltage (e.g., in a range of 900-1100 mV). In some examples, the higher, target voltage may be a voltage at which water molecules are partially or fully dissociated at the oxygen sensor while the base voltage is a voltage at which water molecules are not dissociated at the sensor.
However, the inventors herein have recognized potential issues with operating the exhaust gas sensor in the VVs mode. As one example, air-fuel estimates with the exhaust gas sensor may be invalid when the reference voltage is increased above the base voltage since the oxygen sensor is no longer stoichiometric. For example, at higher reference voltages, the sensor dissociates water vapor and carbon dioxide which contribute to the oxygen concentration represented in the pumping current output by the exhaust gas sensor. Since water vapor and carbon dioxide change with ambient humidity and ethanol concentration in the fuel, and these parameters are unknown, traditional pumping current to air-fuel ratio transfer functions are not accurate at elevated reference voltages. As a result, the vehicle may have to operate in open loop fuel control which may negatively impact emissions, fuel economy, and drivability.
In one example, the issues described above may be addressed by a method for: during operation of an exhaust oxygen sensor in a variable voltage mode where a reference voltage of the oxygen sensor is adjusted from a lower, first voltage to a higher, second voltage, adjusting engine operation based on an air-fuel ratio estimated based on an output of the exhaust oxygen sensor and a learned correction factor based on the second voltage. In other words, a learned correction factor may be used to adjust air/fuel estimates based on outputs of an oxygen sensor when the oxygen sensor operates in a variable voltage mode. As a result, the accuracy of air-fuel ratio estimates while the exhaust oxygen sensor is operating at the higher, second voltage may be increased, thereby increasing the accuracy of engine control based on the estimated air-fuel ratio.
As one example, an exhaust oxygen sensor may operate in a variable voltage mode whereby a reference voltage applied to the oxygen sensor may be adjusted between a lower first voltage where water vapor and carbon dioxide are not dissociated and a higher second voltage where water and/or carbon dioxide are dissociated. A correction factor may be learned based on a difference between a pumping current output by the oxygen sensor when operating at the higher second voltage, and a reference pumping current. The reference pumping current may be based on a known transfer function that relates pumping currents to air/fuel ratios specifically at the second reference voltage. The correction factor may be used to adjust air/fuel ratio estimates when the oxygen sensor operates in a variable voltage mode. In this way, when the exhaust oxygen sensor is operating in the variable voltage mode to determine an additional operating parameter of the engine, air/fuel ratio may also be estimated based on the output of the exhaust oxygen sensor without needing to go into open loop air/fuel control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.